Generally, as electronic parts, such as semiconductors and substrates, have been manufactured to be smaller and thinner, circuits and connection terminals have become very dense and fine. Anisotropic conductive connections have been used in such circuits by dispersing fine conductive balls in an insulating adhesive to obtain a film- or paste-type anisotropic conductive connection material, which is then disposed between connection terminals, heated, and then compressed for adhesion.
Recently, as patterns of connection terminals useful as targets of anisotropic conductive connection become even finer, short circuits between neighboring terminals may occur at the anisotropic conductive connection. Therefore, it is preferable that the conductive balls for anisotropic conductive connection be coated with a thin thermoplastic resin layer or thermosetting resin layer.
However, presently developed insulated conductive balls have diverse problems. For example, when the insulating coating material includes a thermoplastic resin, the coating may be damaged by a solvent during the preparation of the anisotropic conductive connection material, thus preventing the desired insulation performance. Also, crosslinking density is difficult to control when the thermosetting resin is formed into a coating layer. A crosslinking density that is too low causes the same problems as using thermoplastic resins. A very high crosslinking density prevents the coating layer from being removed from the anisotroic connection, thereby preventing current feed between the electrodes.
Examples of the coating process include solution dipping, interfacial polymerization, in-situ polymerization, spray drying, vacuum deposition, physical and mechanical hybridization, etc. However, the above processes suffer from difficulties in forming a uniformly and sufficiently thick insulating coating layer. That is, it is difficult to coat a general resin on surfaces of particles to have a thickness of tens of to hundreds of nm. Japanese Patent Application No. Hei. 8-13076, discloses methods of coating particles with a resin layer, including interfacial polymerization, in-situ polymerization, spray drying, vacuum deposition, etc. In addition, Japanese Patent Application No. Sho. 62-71255 discloses a solution dipping method. However, the above methods do not allow an insulating resin layer having a uniform thickness of tens of to hundreds of nm or more to be easily formed. The method of Japanese Patent Application No. Hei. 8-13076 suffers from agglomeration of particles, and the method of Japanese Patent Application No. Sho. 62-71255 cannot readily form insulating resin layers having a thickness of hundreds of nm. Also, although Korean Patent Application No. 2001-060234 discloses a method of physically attaching crosslinked polymer particles onto conductive balls under gas atmosphere, this method is disadvantageous because it yields a non-uniform coating and weak bonds of particles, resulting in decreased mechanical strength and solvent resistance.
As for the conductive balls coated with thermoplastic resin, the thermoplastic resin film may be damaged by the solvent used for preparation of the anisotropic conductive connection material. Further, limitations are imposed on usable solvent types and mixing compositions. Also, use of the solvent negatively affects the environment including humans.
In addition, processes that include heating and compressing the anisotropic conductive connection cause the coating film to be softened and easily displaced, hence generating a short circuit between the neighboring terminals. Furthermore, for reliable connection of fine circuits in recent years, materials having large quantities of small-sized conductive balls have been used for anisotropic conductive connections. In such cases, since the amount of the thermoplastic resin becomes higher in proportion to an increase in the numbers of conductive balls in the anisotropic conductive connection material, the anisotropic conductive connection material decreases in heat resistance. As the interval between electrode terminals narrows, softened conductive balls are more likely to agglomerate, thereby leading to degraded insulation characteristics.
For conductive balls coated with thermosetting resin, high pressure is applied to anisotropic conductive connection to break the insulating resin layer film of the conductive balls, thereby increasing the probability of electrode terminal damage. In addition, if all of the pieces of the broken film are not recovered, reliable current feed is unachievable.
To solve such problems, Sony Chemical Corp. has proposed the preparation of insulated conductive balls by coating crosslinked polymer particles having a predetermined degree of crosslinkage to conductive balls under gas atmosphere. However, the above preparation process is disadvantageous in terms of non-uniform coating process, and low adhesion between the metal layer and the insulating resin layer due to the use of the not completely crosslinked polymer coating layer. Further, a purifying process should be additionally performed after coating, attributed to inevitable generation of agglomerated particles.